1. Field of the Invention
This invention relates to finder optical systems, and more particularly to finder optical systems suited for single lens reflex cameras or so-called electronic cameras of the TTL optical type using image pickup tubes or solid state image pickup elements such as CCDs.
2. Description of the Prior Art
The development of 35 mm single lens reflex cameras has been based on the use of pentagonal roof-type prisms. The typical finder optical system for a single lens reflex camera is schematically illustrated in FIG. 1, showing a flippable total reflection mirror 101, shutter unit 102, a film plane 103, a focusing screen 104, a pentagonal roof-type prism 105, an eyepiece lens 106, and an observer's eye 107. The optical performance of such a camera is excellent, for the view area compared to the film area, or the field-of-view ratio, is 90% or more. Moreover, with the standard objective lens, the image magnification is 0.8.times. or more. This finder optical system also offers the advantage that the entire body is relatively small in size.
However, in adapting such pentagonal roof-type prisms to optical finder systems of recently developed electronic cameras using so-called CCD or similar image pickup devices, it is difficult to obtain a view and an image magnification equal to those of the conventional single lens reflex cameras. It also becomes difficult to minimize the size of the camera.
This is because:
(i) The effective image frame in the image pickup device of, for example, 2/3 of an inch, measured diagonally, equals about a quarter of the 35 mm film frame. If the conventional pentagonal roof type prism is employed, the length of the optical path is too long to easily obtain a wide view area and a high image magnification;
(ii) The electrical processing circuits occupy a large space in the rear of the image pickup device. This leads to an increase in the distance from the image plane of the photographic lens to the rearmost wall of the camera housing. For this reason, the pupil position of the finder must be shifted a great deal behind the camera. As a result, it becomes difficult to obtain the desired wide view area and the high image magnification;
(iii) The photographic lens must be constructed in the telecentric form for the purpose of color separation. When splitting off part of the light from the photographic lens to the finder, the rays of light greatly diverge, causing a large increase of the size of the reflection mirror; and
(iv) The distance from the beam splitter to the image pickup device must be increased to accommodate the low pass filter, infrared cut filter, and protection glass plate. Thus, the size of the complete camera becomes too large.
For reference, an example of a finder optical system having more than 90% field-of-view ratio using the conventional pentagonal roof type prism in an electronic camera is schematically shown in FIG. 2. The Figure illustrates a photographic lens 200; a beam splitter unit 201; a low pass filter 202; a shutter unit 203; an image receiving surface of the image pickup device 204; a package 205 of the image pickup device with a protection glass layer having an infrared cut effect at the front; a finder 206 including a system for vertically laterally correcting the image; a focusing screen 207; an electrical circuit unit 208 for processing the video signals; and the observer's eye 209.
In general, the higher the image magnification, the easier the finder image is to observe. The image magnification, .gamma., can be expressed terms of the standard focal length f.theta. of the photographic lens and the focal length fe of the eyepiece as .gamma.=f.theta./fe. For an increase of the image magnification .gamma., since the focal length f.theta. of the standard lens has a nearly constant value, it is necessary to decrease the focal length fe of the eyepiece. Because the eyepiece is arranged so that its front focal point lies near the focusing screen in the finder optical system, the length of the optical path of the correct image forming optical system from the focusing screen to the eyepiece must be as short as possible. Now, assuming that the image pickup device is 2/3 inches with the standard lens having a focal length f.theta.=12.5 mm, when the image magnification is taken at .gamma.=0.5, the value of the focal length fe is found to be 25 mm.
To obtain a high view area ratio, the focusing screen must be as large in size as the effective image pickup area. Moreover, the correct image forming system must be made large enough to create the optical path that permits observation of the entire area of the focusing screen.
In FIG. 2, to obtain a view area of at least 90% with an image magnification of at least .gamma.=0.5, the length of the optical path from the focusing screen to the front principal plane of the eyepiece must be made almost equal to the focal length of the eyepiece, for instance, 25 mm. Secondly, all rays of light from the focusing screen must enter the eyepiece. A pentagonal roof type prism to fulfill these requirements is possible. As shown in FIG. 2, however, the eyepiece must be placed farther away from the eye 209, because the prism 210 is positioned at the most forward point in the finder unit 206. The eyepiece 211 must also be arranged adjacent the exit face of the prism 210. The eye is placed at a distance almost equal to the focal length of the eyepiece from the rear principal point thereof. As shown in FIG. 2, the eye 209 is spaced apart from the eyepiece 211. The interval between the principal points of the eyepiece thus has to be very long. This is very difficult to realize.
As will be understood from the foregoing, using the prior known prism in the finder optical system of the electronic camera whose effective image pickup area is relatively small, makes it very difficult to achieve a high view area ratio and a high image magnification.